1. Technical Field
The present invention relates to an ink jet printer that discharges minute liquid ink droplets having a plurality of colors from a plurality of nozzles to form fine particles (ink dots) on a printing medium, thereby printing a predetermined character or image.
2. Related Art
In general, such an ink jet printer is inexpensive and can easily produce a high-quality color print out. With the popularization of personal computers and digital cameras, ink jet printers have come into widespread use in the home as well as in the office.
Generally, in such an ink jet printer, nozzles of printing heads (which are also called ink jet heads) discharge (eject) liquid ink droplets onto a printing medium while a moving body, which is called a carriage composed of ink cartridges and the printing heads, reciprocates over the printing medium in a direction orthogonal to the direction in which the printing medium is transported, thereby forming minute ink dots on the printing medium. In this way, the ink jet printer prints a desired character or image on the printing medium to produce a desired print out. The carriage is provided with four color (black, yellow, magenta, and cyan) ink cartridges and printing heads corresponding to the four colors, which makes it possible to easily perform full color printing using the four colors as well as monochrome printing (six, seven, or eight color printing including black, yellow, magenta, cyan, light cyan, and light magenta has also been put to practical use).
In the ink jet printer that performs printing while reciprocating the ink jet heads on the carriage in the direction (in the width direction of the printing medium) orthogonal to the direction in which the printing medium is transported, in order to completely print one page, it is necessary to reciprocate the ink jet heads a number of times ranging from several tens of times to one hundred times or more. In contrast, in an ink jet printer that uses long ink jet heads having a length equal to the width of a printing medium without using the carriage, it is not necessary to move the ink jet heads in the width direction of the printing medium, and only one pass is required to print the printing medium, which makes it possible to perform high-speed printing, similar to an electrophotographic printer. The former is generally called a ‘multi-pass ink jet printer’, and the latter is generally called a ‘line head ink jet printer’.
However, when aqueous ink is used for the ink jet printer, the printing medium is curved after printing, that is, so-called curling occurs in the printing medium after printing. In order to prevent the curling, JP-A-10-151733 discloses an ink jet printer in which a roller comes into contact with the surface of the printing medium opposite a printing surface of the printing medium immediately after printing to apply a curl-preventing liquid onto the printing medium, and the printing medium passes through a heat roller heater having heating sources at the upper and lower parts thereof to be dried. In addition, JP-A-2005-178251 discloses an ink jet printer in which, in order to prevent the curling, a vapor generating device for generating vapor using heat or ultrasonic vibration is provided inside a roller for transporting a printing medium, and the roller comes into contact with the printing medium to supply vapor from the surface of the roller to the surface of the printing medium opposite the printing surface of the printing medium. Further, JP-A-2005-178252 discloses a technique for detecting humidity in a printing environment and controlling the amount of vapor generated according to the environmental humidity in the ink jet printer disclosed in JP-A-2005-178251.
However, in the ink jet printers disclosed in JP-A-10-151733, JP-A-2005-178251, and JP-A-2005-178252, the roller comes into contact with the printing medium to supply a liquid, such as the curl-preventing liquid, or vapor. However, a printing medium containing a large amount of liquid is likely to be closely adhered to the roller, that is, the detachability between the roller and the printing medium is lowered, which may result in an error in the transport of a printing medium.